The invention relates to a key for a security barrel, useful in particular for the operation of a lock or of a bolt, the key being of the type in which the body of the key is intended to enter a recess of the barrel rotor and is provided with at least one translatable movable plunger able to be displaced, with a movement of limited amplitude, in a recess provided in the key, the plunger being intended to co-operate with a positioning member provided in the rotor recess so as to project outwardly from the recess and to push a movable bolt of the rotor into a position permitting rotation of the barrel rotor relative to the stator.